1. Field of the Invention
The invention relates to a vehicle and method for controlling power to wheels in a vehicle.
2. Background Art
In recent years, automobile manufacturers have developed vehicles powered at least in part by electric power sources. Electric vehicles generally fall into one of three categories: pure electric vehicles (EVs), hybrid electric vehicles (HEVs), and fuel cell electric vehicles (FCEVs). Each of these three vehicle types can be propelled by one or more electric motors, which in turn, receive power from an energy storage device, such as a battery or a capacitor.
Unlike gasoline, which contains the same amount of energy regardless of the amount of gasoline stored in the vehicle, the available energy and power of a battery vary depending upon the condition of the battery, e.g., battery state of charge, battery temperature, and the like. Accordingly, it is desirable that the power draw from the battery be controlled to ensure correct operation of the vehicle.
One method of regulating battery power draw is by using a motor control system that establishes an upper limit under which the driver power demand, i.e., torque demanded by the vehicle operator, must reside. By limiting the value of the driver power demand to less than a predetermined limit, the motor control system attempts to maintain the battery voltage within a range that is sufficient to prevent the electric motor (or motor inverter) from shutting down due to an over discharge condition.
One example of a system and method for motor control is disclosed in U.S. Patent Application Publication No. 2004/00364434 to Chen et al. (“Chen”). Chen discloses a closed loop permanent magnet motor control method and system to optimally partition torque and flux-weakening currents to produce a desired torque without exceeding the capabilities of the DC power source. According to the Chen system, if the demanded torque current is higher than the maximum torque current limit, then the magnitude of the flux current is increased by adding together the peak-torque-per-amp component of the flux current and the output of a proportional-plus-integral (PI) controller. The PI controller operates on the difference between the demanded torque current and the commanded torque current, and ensures that the demanded torque current does not exceed the maximum torque current limit. In this manner, the motor can produce torque at the maximum capability of the motor while transitioning into and out of the flux-weakening region.
One limitation of the approach disclosed by Chen is that the maximum demand limit, i.e., Lmax, is difficult to estimate accurately. As such, the driver power demand signal may not be limited properly, causing the battery to be operated out of its operating range. In addition, this approach does not address battery over discharge conditions that result from inaccuracies in the motor torque control mechanism. Since inaccuracies in the control mechanism may result in a greater battery power draw than would normally be associated with a given power demand signal, a battery over discharge condition may result even though the power demand signal was determined to be within the limits. Similarly, the inability to accurately predict additional power demands necessitated by losses in the system may result in a battery over discharge condition, even though the power demand signal was within the demand limits.
Accordingly, it would be desirable to have a vehicle and method for controlling the power to the wheels of a vehicle such that battery over discharge occurrences are reduced or eliminated.